1. Field of the Invention
The present invention relates to a method and apparatus for clearing electrical shorts and more particularly to a method and apparatus for clearing deposits of conductive or semiconductive flakes from a high-voltage electrode.
2. Background and Description of the Prior Art
Sputtering is a process for depositing a coating onto a substrate. Typically, during a sputtering process, an inert gas is introduced into a vacuum chamber containing an anode, a substrate, and a target cathode. A relatively high dc voltage is applied across the anode and target cathode. The inert gas molecules are ionized to form a plasma. The ionized gas molecules strike the target cathode which causes atoms and/or molecules of the target cathode to be sputtered from the target cathode surface. The substrate is positioned to intercept this sputtered target material which forms an adherent coating upon striking the substrate. Sputtering is a generally line-of-sight coating process.
A sputtering apparatus for making metal films, such as electrical resistors or electrodes of electrical capacitors, is disclosed in U.S. Pat. No. 4,080,281. In the apparatus described in that patent, the substrates are placed in an annular cylindrical cage which serves as the anode. An outer coaxial, annular cylindrical target cathode circumferentially surrounds the cage to sputter radially inward on the cage from all directions. An inner coaxial, rod target cathode lies along the cage's longitudinal axis to sputter radially outwardly from all directions. The cage rotates to cause the metal films to be more uniform.
U.S. Pat. Nos. 3,562,140 and 3,632,494 disclose sputtering apparatus for coating razor blades with chromium.
Sputtering devices may also be used to coat substrates such as nuclear fuel pellets. As is known to those skilled in the art of designing nuclear fuel assemblies, the life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. Burnable absorbers are materials (such as zirconium diboride, boron, gadolinium, samarium, europium, and the like) which have a high probability (or cross section) for absorbing neutrons, resulting in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons, all while producing no new or additional neutrons. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly. During reactor operation, the effect of the burnable absorbers becomes progressively reduced resulting in a longer fuel assembly life at a relatively constant fission level. Longer fuel assembly life means less frequent nuclear reactor fuel assembly replacement which is costly and time consuming process.
U.S. Pat. No. 3,427,222 discloses fusion bonding a burnable poison (also known a burnable absorber) layer to the surface of a nuclear fuel pellet substrate.
Commonly assigned U.S. patent application Ser. No. 526,712, now U.S. Pat. No. 4,587,088, ("Coating a Nuclear Fuel with a Burnable Poison" by Kenneth C. Radford, filed Aug. 26, 1983) discloses sputtering as a method for depositing a layer of burnable poison on nuclear fuel pellets.
Commonly assigned U.S. patent application Ser. No. 623,747 now U.S. Pat. No. 4,560,462 ("Apparatus for Coating Nuclear Fuel Pellets with a Burnable Absorber") by Kenneth C. Radford et al., filed June 22, 1984, discloses an apparatus for sputter coating nuclear fuel pellets.
Occasionally, during the sputtering process, a flake of accumulated burnable neutron absorber which was deposited on a surface other than a fuel pellet spalls off and falls onto one of the sputtering cathodes. That flake will constitute a resistive conductor which may short out the high voltage to that cathode and quench the plasma of vaporized neutron absorber in the vacuum chamber.
In a typical sputtering chamber, the dc power source can supply up to about 17 amperes, at approximately 480 volts. A minimum of about 200 volts is required to start and/or maintain a sputtering plasma. Whenever a flake of burnable neutron absorber, such as zirconium diboride, shorts the cathode to ground, the voltage will typically fall to anywhere from zero up to 25 volts D.C., while the current between the cathode and ground will rise to the maximum current of the power supply, typically about 17 amperes. While that small amount of power may make the flake glow, so that its presence can be visually confirmed, only very rarely will it generate sufficient heat to clear the short circuit. In this regard, it is noted that shorting flakes are typically large and thick and therefore difficult to evaporate. Therefore, in a typical prior art sputtering chamber, the presence of a shorting flake between the cathode and ground requires that the chamber be shut down and cleaned out. Since the sputtering chamber operates at a very high vacuum, on the order of 30 microtorr, it normally takes as long as six hours to cool down the sputtering apparatus, open up the vacuum chamber, clean out the system, re-pumpdown the vacuum chamber down to 30 microtorr and then apply the appropriate voltage profile to recreate a high voltage sputtering plasma.